Sonic well logs are typically derived from a tool suspended in a borehole or well by a cable. The sonic tool will often have a sonic transmitter and a plurality of receivers. Typically, a sonic signal is transmitted from one longitudinal end of the tool and is received (detected) at the other end. The sonic signal from the transmitter enters the formation adjacent the borehole, and the arrival times are used to find formation parameters. Sonic measurements are taken every few inches as the tool is moved in the borehole or well. In an LWD tool, the measurements may be taken as the tool drills into the borehole or as the tool is tripped into or out of the borehole.
Various data processing techniques are known for taking the sonic measurements and generating useful information. As disclosed in co-owned U.S. Pat. No. 4,594,691 to Kimball et al., a “slowness time coherency” (STC) technique may be utilized to stack moveout-corrected receiver outputs by depth level and identify selected peaks of a coherence measure of the signal. The packets of coherent energy can then be identified in terms of their origin such as compressional, shear, Stoneley or other arrivals. Various logs such as a “slowness log” can be generated, and the slowness information can be used to provide information regarding formation parameters such as porosity.
One complication involved in making sonic measurements and generating useful information is the introduction of noise. It is known that the tool body itself introduces noise by conducting energy directly from the acoustic source to the detectors. In order to reduce the impact of noise, some wireline logging tools have been provided with slots, perforations or grooves in the tool housing so as to create a tortuous path for the sonic energy, thereby attenuating and/or slowing down the acoustic signal being transmitted by the housing. Other tools have mounted the transmitters and receivers on flexible material such as rubber or on other material whose acoustic velocity is slower than the housing material. For LWD tools, not only is noise introduced by the direct path of the housing, but considerable noise (interference) is introduced by the interaction of the drill bit with the formation. In order to reduce the noise, and as disclosed in co-owned U.S. Pat. No. 5,852,587 to Kostek et al., acoustic energy is transmitted preferentially at or near a “stop band” frequency. In addition, a plurality of axially periodic, substantially circumferentially continuous sections of the drill collar with acoustic propagation characteristics different from the drill collar may be provided in such a manner that the acoustic energy at a stop band frequency is further attenuated or filtered through a combination of reflection and destructive interference.